For ecological and toxicological reasons, but also to minimize the so-called fogging effect (deposition of thin but highly light-refracting films on the insides of glass panels/windscreens by heat-induced migration of volatile substances from the used materials) caused by plastics parts incorporated in automobile interiors, the thorough removal of volatile constituents such as residual monomers, chain regulators or low molecular weight reaction products such as recombination or reaction products of the employed polymerization initiator is becoming increasingly important also in the production of polymer-filled polyether polyols (“polymer polyols”; “PMPO”). In particular as regards the European market, it is at the same time important that the polymer polyols do not in this connection suffer from too serious a discoloration (yellowness index <25, measurement according to DIN 5033, calculation according to DIN 6167 and ASTM E 313). For this reason none of the normally employed methods, such as for example the use of thin film evaporators, is possible, because temperatures of >160° C. are necessary to achieve the desired separation result.
It is known from WO 00/00531 and WO 98/52988 that polymer polyols are subjected to a purification step after the free-radical polymerization of the unsaturated monomers used for the filling and after a post-reaction time. Vacuum stripping is suggested for this purpose, although no specific procedure or process conditions are given.
The purification and deodorization of polyether polyols and aqueous polymer dispersions is however well described in the prior art. The processes described there, such as vacuum stripping (vacuum flashing), vaporization in downpipe apparatus, the use of thin-film evaporators or the use of floor, filled and packed columns and their conditions of use cannot however be directly extrapolated to polymer polyols, as the substances to be separated are essentially different compounds. Furthermore polymer polyols under the same operating conditions and temperatures up to 160° C. exhibit significantly higher viscosities of up to 500 mPas than polyether polyols (up to 60 mPas) and aqueous polymer dispersions (up to 10 mPas).
Patent Application DE-A-198 28 183 is concerned with the stripping of aqueous polymer dispersions and suspensions in which the removal of volatile constituents by chemical deodorization, followed by a physical deodorization preferably with steam in countercurrent columns is described. The removal of monomers and low-boiling components by physical deodorization is also described in Patent Application DE-A 197 16 373. This is preferably carried out with steam on weep-type or cross-current perforated trays, the structural details of which are disclosed in the patent. Countercurrent columns with weep-type or cross-current perforated trays for the removal of undesired organic components are also described in DE-C 198 47 115. Preferably gaseous substances, particularly preferably steam, are used as stripping agents.
Patent Application EP-A 0 982 341 describes a process for the production of polyether polyols, in which these are worked up by a combination of reactive-distillative steps. The distillative step consists of a combination of atmospheric pressure columns and vacuum columns operated with inert gas, preferably nitrogen.
In EP-A 0 819 712, a process for the production of polyurethane flexible foams is described. In this case the purification of polyols is carried out in a combined thermal-distillative step in surface-enlarging devices, optionally using inert gas. A characteristic features is that the treatment is carried out with a maximum 0.1 wt. % of water, preferably in the anhydrous state.
In EP-A 0 864 598, a thermal-distillative treatment step is described for polyether polyols by employing a countercurrent column and using a liquid and/or gaseous purification agent that is highly insoluble in the polyether polyol. CO2 and nitrogen are mentioned as purification agents.
In U.S. Pat. No. 6,060,627, the removal of secondary products from a polyol (propoxylated glycerol) in a countercurrent column with ordered packings is described. The secondary products allyl alcohol, propylene glycol as well as monoethers and diethers of allyl alcohol are removed from the polyol by using steam or nitrogen as stripping agent.
Neither a stripping of pure polyether polyols nor polymer-filled polyether polyols in packed columns using steam as stripping agent is therefore known from the prior art.